1. Field of the Invention
This invention relates to an ink jet printing system and more particularly to a thermal drop-on-demand ink jet printing system.
2. Description of the Prior Art
A thermal drop-on-demand ink jet printing system is known in which a heater is selectively energized to form a "bubble" in the adjacent ink. The rapid growth of the bubble causes an ink drop to be ejected from a nearby nozzle. Printing is accomplished by energizing the heater each time a drop is required at that nozzle position to produce the desired printed image.
One of the most significant failure mechanisms in a thermal drop-on-demand ink jet printing system is the erosion caused by bubble collapse after the drive pulse, which energizes the heater, is turned off. During this phase, the condensation of vapor usually produces a very high speed implosion which sends fairly high intensity shock waves to the heater surface. These waves are termed cavitational shock. Even though a passivation layer protects the top surface of the heater, in time the cavitational shock erodes the protective layer which leads to damage to the heater element and eventual failure.
One way in which the problem of cavitation shock damage has been addressed is described in U.S. Pat. No. 4,514,741 to Meyer. Meyer shows a thermal bubble jet printer in which the heater element comprises a resistive region having a conductive region at its center. The conductive region effectively electrically shorts the underlying area of the heater element and enables the production of a toroidally shaped bubble. The toroidally shaped bubble is described as fragmenting during collapse, thereby randomly distributing the resultant acoustic shock across the surface of the heater element to minimize cavitation damage. While the design may reduce cavitation damage, it is less efficient since there is no bubble in the direction of the associated nozzle whereas this direction is where the maximum pressure wave is desired.
U.S. Pat. No. 4,317,124 to Shirato et al shows a drop-on-demand ink jet printing system which utilizes a pressurized system to produce leakage of ink from the nozzles, and an ink intake, in the vicinity of the nozzle, to remove the ink not used for printing. A transducer is energized with the information signals to eject a drop of ink from the nozzle when needed for printing. One embodiment is shown in FIG. 28 which was used to gain experimental data on the optimum width of the heaters for a thermal transducer. Two spaced heaters are shown and these heaters are connected in a series electrical circuit.
European Patent Application No. 84302524.8 shows a thermal bubble jet printer in which two elongated resistive elements are spaced apart and connected in a series electrical circuit to produce a bubble for forming a drop for printing. The shape of the resulting bubble is not described, but in FIG. 5 the bubble is shown collapsing in the area between the two resistive elements.
Published unexamined Japanese Patent Application No. 59-138460 describes a thermal bubble jet printer having a partition wall near the heater surface shaped to make the flow of ink, during replenishment of ink after the emission of a drop, unbalanced in the vicinity of the heater so that the impact generated by the collapsing bubble is shifted to a position away from the heater surface to avoid damage to the heater.
No prior art is known in which a pillow-shaped bubble is formed with high pumping efficiency, and in which the bubbles collapse in an area enclosed by the heater structure so that erosion damage can be greatly reduced or even eliminated.